Debt - Narrative (Details)					9 Months Ended
	Apr. 30, 2020 USD ($)	Apr. 23, 2020 USD ($)	Apr. 09, 2020 USD ($)	Oct. 02, 2019 USD ($)	Jun. 30, 2020 USD ($) credit_facility	Apr. 01, 2020 USD ($)	Sep. 30, 2019 USD ($)
Line Of Credit Facility [Line Items]
Maximum borrowing capacity					$ 2,200,000,000
Maximum debt-to-total-capitalization ratio					70.00%
Debt-to-total-capitalization ratio					0.43
Minimum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio					50.00%
Outstanding indebtedness					$ 15,000,000
Maximum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio					60.00%
Outstanding indebtedness					$ 100,000,000
Revolving Credit Facility
Line Of Credit Facility [Line Items]
Number of credit facilities | credit_facility					4
$200 Million Term Loan Agreement
Line Of Credit Facility [Line Items]
Debt agreement term			2 years
Unsecured 2.625% Senior Notes, due 2029
Line Of Credit Facility [Line Items]
Debt instrument, face amount				$ 300,000,000
Interest rate				2.625%	2.625%
Effective interest rate					2.72%
Unsecured 3.375% Senior Notes, due 2049
Line Of Credit Facility [Line Items]
Debt instrument, face amount				$ 500,000,000
Interest rate				3.375%	3.375%
Debt issuance cost				$ 791,700,000
Effective interest rate					3.42%
Five Year Unsecured Revolving Credit Agreement
Line Of Credit Facility [Line Items]
Outstanding commercial paper					$ 0		$ 464,900,000
Five Year Unsecured Revolving Credit Agreement | Commercial Paper
Line Of Credit Facility [Line Items]
Maximum borrowing capacity					1,500,000,000
Accordion feature					250,000,000
Maximum borrowing capacity post accordion feature					$ 1,750,000,000
Debt agreement term					5 years
$25 Million Bank Loan Agreement | Line of Credit
Line Of Credit Facility [Line Items]
Maximum borrowing capacity					$ 25,000,000
Debt agreement term					364 days
$50 Million Bank Loan Agreement | Line of Credit
Line Of Credit Facility [Line Items]
Maximum borrowing capacity						$ 50,000,000
Outstanding borrowings					$ 0
$10 Million Revolving Credit Note | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity					$ 10,000,000
Debt agreement term					364 days
$50 Million Revolving Credit Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	$ 50,000,000				$ 50,000,000
Debt agreement term	364 days
Remaining borrowing capacity					44,400,000
$50 Million Revolving Credit Facility | Line of Credit
Line Of Credit Facility [Line Items]
Outstanding borrowings					0
$600 Million Revolving Credit Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		$ 600,000,000
Debt agreement term		364 days
Outstanding borrowings					$ 0
LIBOR | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread					0.00%
LIBOR | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread					1.25%
LIBOR | $600 Million Revolving Credit Facility | Revolving Credit Facility | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread		0.00%
LIBOR | $600 Million Revolving Credit Facility | Revolving Credit Facility | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread		1.25%
Secured Debt | $200 Million Term Loan Agreement
Line Of Credit Facility [Line Items]
Debt instrument, face amount			$ 200,000,000
Secured Debt | LIBOR | $200 Million Term Loan Agreement
Line Of Credit Facility [Line Items]
Interest rate spread			1.25%